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Abstract Observations of seismic waves that have passed through the Earth's lowermost mantle provide insight into deep mantle structure and dynamics, often on relatively small spatial scales. Here we use SKS, S2KS, S3KS, and PKS signals recorded across a large region including the United States, Mexico, and Central America to study the deepest mantle beneath large swaths of North America and the northeastern Pacific Ocean. These phases are enhanced via beamforming and then used to investigate polarization‐ and propagation direction‐dependent shear wave speeds (seismic anisotropy). A differential splitting approach enables us to robustly identify contributions from anisotropy. Our results show strong seismic anisotropy in approximately half of our study region, indicating that anisotropy may be more prevalent than commonly thought. In some regions, the anisotropy may be induced by flow driven by sinking cold slabs, and in other, more compact regions, by upwelling flow. Measured splitting due to lowermost mantle anisotropy is sufficiently strong to be non‐negligible in interpretations of SKS splitting due to upper mantle anisotropy in certain regions, which may prompt future re‐evaluations of upper mantle anisotropy beneath North and Central America.
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Investigating Ultra‐Low Velocity Zones as Sources of PKP Scattering Beneath North America and the Western Pacific Ocean: Potential Links to Subducted Oceanic Crust
Abstract Seismic energy arriving before the compressional (P) wave passing through the core (PKP), called PKP precursors, have been detected for decades, but the origin of those arrivals is ambiguous. The largest amplitude arrivals are linked to scattering at small‐scale lowermost mantle structure, but because these arrivals traverse both source and receiver sides of the mantle, it is unknown which side of the path the energy is scattered from. To address this ambiguity, we apply a new seismic array method to analyze PKP waveforms from 58 earthquakes recorded in North America that allows localization of the origin of the PKP precursors at the core‐mantle boundary (CMB). We compare these measurements with high frequency 2.5‐D synthetic predictions showing that the PKP precursors are most likely associated with ultra‐low velocity zone structures beneath the western Pacific and North America. The most feasible scenario to generate ULVZs in both locations is through melting of mid‐ocean ridge basalt in subducted oceanic crust.
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- PAR ID:
- 10532272
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- AGU Advances
- Volume:
- 5
- Issue:
- 4
- ISSN:
- 2576-604X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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